June 2008 - Haskell-Cafe

ICFP2008 Call for Poster proposals
by Matthew Fluet (ICFP Publicity Chair) 04 Jun '08

04 Jun '08

ICFP 2008 poster session September 21, 2008 Call for presentation proposals ICFP 2008 will feature a poster session for researchers and practitioners, including students. The session will provide friendly feedback for work that is in gestation or ongoing, as well as opportunities to meet each other and exchange ideas. We welcome poster submissions on all ICFP topics, especially presentations of - applications of and to functional programming; - recent work presented at more distant venues; and - ongoing work, whether or not submitted to ICFP. There will be no formal proceedings, but presenters will be invited to submit working notes, demo code, and other materials to supplement their abstract and poster. These materials will be released informally on a Web page dedicated to the poster session. An accepted submission is not intended to replace conference or journal publication. Persons interested in presenting a poster are invited to submit a one-page abstract in SIGPLAN conference style http://www.acm.org/sigs/sigplan/authorInformation.htm to the Web site https://www.softconf.com/s08/icfp08-posters/submit.html by June 30, 2008. The program committee will review the submissions for relevance and interest, and notify the authors by July 14, 2008. Accepted posters must be presented by the authors in person on Sunday, September 21, 2008. Important dates: Submission: Monday, June 30, 2008 Notification: Monday, July 14, 2008 Presentation: Sunday, September 21, 2008 Program committee: Benjamin Pierce (University of Pennsylvania) Colin Runciman (University of York) Chung-chieh Shan (Rutgers University)

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[ANNOUNCE] git-darcs-import 0.1
by Bertram Felgenhauer 04 Jun '08

04 Jun '08

Hi, I'm pleased to announce yet another tool for importing darcs repositories to git. Unlike darcs2git [1] and darcs-to-git [2], it's written in Haskell, on top of the darcs2 source code. The result is a much faster program - it can convert the complete ghc 6.9 branch (without libraries) in less than 15 minutes on my slightly dated machine (Athlon XP 2500+), which is quite fast [3]. Incremental updates work, too. The program is still rough around the edges, and there's some cosmetical work to do, especially with respect to converting author names. The program should recover from most errors, as long as nobody else modifies the destination repository. Nevertheless, it seems quite useable already. I hope somebody finds this useful. You can grab the source at http://int-e.home.tlink.de/haskell/git-darcs-import-0.1.tar.bz2 Look at the README for further information. Credits go to: David Roundy and all contributors for darcs2. The code base is surprisingly pleasant to work with. And of course, Linus Torvalds, Junio Hamano and all other git contributors. Enjoy, Bertram [1] http://repo.or.cz/w/darcs2git.git?a=shortlog [2] http://git.sanityinc.com/?p=darcs-to-git.git [3] http://nominolo.blogspot.com/2008/05/thing-that-should-not-be-or-how-to.html

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Installing Haskore under Windows
by David 04 Jun '08

04 Jun '08

Hi! I'm currently running ghc under Windows XP and want to play around with Haskore, but I'm a little confused about how to install it, or if it's even possible to use it with ghc. The 'readme' file contains this note: "Note that the file ghc_add/IOExtensions.hs is a partial replacement of a library file of the same name under Hugs. It does not work yet on Windows/GHC (where one has to distinguish between binary and text IO)." Does this mean I have to use Hugs rather than ghc? I have both Hugs and ghc installed, but I'd prefer to use ghc if possible. In either case, I find the install instructions (for Haskore, that is) a little confusing, probably because I'm also new to Haskell. Any help would be greatly appreciated. Thanks.

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problems with derive/TH
by Thomas Hartman 04 Jun '08

04 Jun '08

Haskellers/Chris, As part of my quest to better understand STM and eventually build a concurrent haskell server app, I checked out an embryonic haskell mud demo from Chris Smith's repo. It seemed like a great sample app. But I ran into a build issue that seems to have to do with derive/TH. I am assuming this used to work, but something changed, either in TH itself or switching from ghc6.6 to ghc6.8. From the error message (below) I surmise either a) M.Map based values used to automagically work with derive/TH, but no longer do... in which case I need to create this instance manually or b) this should still work, more or less... but I have to import the map instance via an additional module, or something like that. Any advice? Thomas. ***** This is the error I am getting: thartman@thartman-laptop:~/mud>cat _darcs/prefs/defaultrepo http://cdsmith.twu.net/demos/mud thartman@thartman-laptop:~/mud>ghc --version The Glorious Glasgow Haskell Compilation System, version 6.8.2 thartman@thartman-laptop:~/mud>runghc Setup.lhs configure; runghc Setup.lhs build Configuring mud-0.1... Preprocessing executables for mud-0.1... Building mud-0.1... [2 of 5] Compiling World ( World.hs, dist/build/mud/mud-tmp/World.o ) World.hs:42:0: No instances for (Eq (a (M.Map String Player)), Eq (a (M.Map ItemId Item)), Eq (a (M.Map PlayerId Player)), Eq (a (M.Map RoomId Room)), Eq (a RoomId)) arising from the 'deriving' clause of a data type declaration at World.hs:(42,0)-(50,14) Possible fix: add an instance declaration for (Eq (a (M.Map String Player)), Eq (a (M.Map ItemId Item)), Eq (a (M.Map PlayerId Player)), Eq (a (M.Map RoomId Room)), Eq (a RoomId)) When deriving the instance for (Eq (World a))

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A simple beginner question
by Adam Smyczek 04 Jun '08

04 Jun '08

Example: data SampleType = A | B Int | C String | D -- .... etc. sampleTypes = [A, B 5, C "test"] :: [SampleType] How do I find for example element A in the sampleTypes list? Do I have to create e.g.: isA :: SampleType -> Bool isA A = True isA _ = False for every constructor and use find? It feels like this is not the quicker method. Thanks, Adam

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Wouter-style expressions
by Ron Alford 04 Jun '08

04 Jun '08

Here's something that should be an easy extension of Wouter's approach to extensible datatypes, but I'm failing (possibly since it's 2:20am). I several classes of expressions I'm trying to represent (thus, Wouter's approach), and my first operation to implement over them is printing. Attached is a simplified version of what I'm doing. Expressions are composed of conjunctives (and) and atoms. Atoms are composed of a predicate string and a list of fillers (taken from 'Const' and 'Var'). For example (in a lisp like syntax): (and (pred c ?var1) (pred ?var1 ?var2)) To do this, I defined a 'Printable' class: class Functor f => Printable f where exprDoc :: f t -> Doc I think the 't' here will get me into trouble later. Combinations of printable types are also printable: instance (Printable f, Printable g) => Printable (f :+: g) where exprDoc (Inr x) = exprDoc x exprDoc (Inl y) = exprDoc y Constants, variables, and atoms are defined to be printable: instance Printable Var where exprDoc (Var name) = text ('?':name) instance Printable Const where exprDoc (Const name) = text name instance Printable f => Printable (Atomic (Expr f)) where exprDoc (Atomic p tl) = parens $ hsep $ (text p) : (map (\ (In t) -> exprDoc t) tl) But the obvious definition for conjunction doesn't work: instance Printable And where exprDoc (And el) = sep (map exprDoc el) GHC barfs, throwing: Couldn't match expected type `f t' against inferred type `t1' `t1' is a rigid type variable bound by the type signature for `exprDoc' at WouterTest.hs:62:17 Expected type: [f t] Inferred type: [t1] In the second argument of `map', namely `el' In the first argument of `sep', namely `(map exprDoc el)' I've attached the code. Compile and inspect with: $ ghci -fglasgow-exts -fallow-overlapping-instances WouterTest.hs test1 works great (other than needing some redundant typing, any hints?). test2 needs the definition of 'Printable And' to print, but I haven't gotten that to work yet. It also needs the redundant typing. -Ron However, I tried making more complex expression (conjunction, in this case), but I can't get the types to align. In particular, if I uncomment the obvious definition

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[ANN] bloomfilter 1.0 - Fast immutable and mutable Bloom filters
by Bryan O'Sullivan 03 Jun '08

03 Jun '08

I'm pleased to announce the availability of a fast Bloom filter library for Haskell. A Bloom filter is a probabilistic data structure that provides a fast set membership querying capability. It does not give false negatives, but has a tunable false positive rate. (A false positive arises when the filter claims that an element is present, but in fact it is not.) The library is easy to use. As an example, here's a reimplementation of the Unix "spell" command. import Data.BloomFilter.Easy (easyList, elemB) main = do filt <- (easyList 0.01 . words) `fmap` readFile "dictionary.txt" let check word | word `elemB` filt = "" | otherwise = word ++ "\n" interact (concat . map check . lines) It is also carefully tuned for performance. On my laptop, I can sustain a construction or query rate well in excess of a million ByteStrings per second. Source code: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/bloomfilter Latest bits: darcs get http://darcs.serpentine.com/bloomfilter <b

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ghc 6.8.2 issue
by Galchin, Vasili 03 Jun '08

03 Jun '08

Hello, I have been developing new code for the "unix" package. I have run into what I think are ghc 6.8.2 anomalies but couldn't see a pattern. Possibly now I do. I have been using the 32-bit x86 ghc binary that I downloaded from http://www.haskell.org and running on Ubuntu Linux. I am pretty I can repeat now. Everything is ok until I do "sudo runhaskell Setup.hs install" which installs a new "unix" package that I just built. Of course the "intall" installs into /usr/local/lib/unix-2.3.0.0/ghc-6.8.2. The "install" seems to destroy some ghc compiler state under /usr/local ... This is of course just a theory. However, once I re-install ghc et. al. from the binary download then things are ok again. I have tried the ghc re-install at least 4 times. Hope I am not misleading anybody .. just trying to help! Kind regards, Vasili

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hs-plugins compile error
by Edsko de Vries 03 Jun '08

03 Jun '08

Hi, I'm getting the compilation error that is actually logged on Hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/plugins Below is a small diff file that resolves these problems; I don't know what the proper protocol is for submitting these diffs but it may be useful to someone. - E diff -ur plugins-1.2-orig/src/System/Plugins/Env.hs plugins-1.2/src/System/Plugins/Env.hs --- plugins-1.2-orig/src/System/Plugins/Env.hs 2008-06-02 14:57:59.000000000 +0100 +++ plugins-1.2/src/System/Plugins/Env.hs 2008-06-02 15:00:25.000000000 +0100 @@ -73,7 +73,7 @@ import Control.Concurrent.MVar ( MVar(), newMVar, withMVar ) -import Distribution.Package +import Distribution.Package hiding (packageName) import Text.ParserCombinators.ReadP import qualified Data.Map as M diff -ur plugins-1.2-orig/src/System/Plugins/PackageAPI.hs plugins-1.2/src/System/Plugins/PackageAPI.hs --- plugins-1.2-orig/src/System/Plugins/PackageAPI.hs 2008-06-02 14:57:59.000000000 +0100 +++ plugins-1.2/src/System/Plugins/PackageAPI.hs 2008-06-02 14:59:49.000000000 +0100 @@ -40,7 +40,7 @@ #if CABAL == 1 || __GLASGOW_HASKELL__ >= 604 import Distribution.InstalledPackageInfo -import Distribution.Package +import Distribution.Package hiding (depends, packageName) #else import System.Plugins.Package #endif diff -ur plugins-1.2-orig/src/System/Plugins/ParsePkgConfCabal.hs plugins-1.2/src/System/Plugins/ParsePkgConfCabal.hs --- plugins-1.2-orig/src/System/Plugins/ParsePkgConfCabal.hs 2008-06-02 14:57:59.000000000 +0100 +++ plugins-1.2/src/System/Plugins/ParsePkgConfCabal.hs 2008-06-02 14:58:56.000000000 +0100 @@ -6,7 +6,7 @@ ) where import Distribution.InstalledPackageInfo -import Distribution.Package +import Distribution.Package hiding (depends) import Distribution.Version import Data.Char ( isSpace, isAlpha, isAlphaNum, isUpper, isDigit )

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Rewrite class with fundeps in pure h98?
by Darrin Thompson 03 Jun '08

03 Jun '08

I thought that I should be able to write something like the toHtml function below with pure H98, but when I went to write it I ended up needing fundeps. Is there a way to express this without using any extensions? The idea was to take a remotely Dom-like tree and flatten it into a string, but not require any particular string type. Mostly I'm ripping off HStringTemplate for the fun of it. {-# OPTIONS -fglasgow-exts #-} module Control.WebWidget where import Data.Monoid import Data.String data (IsString s, Monoid s) => Dom s = Element s [Attr s] [Dom s] | Text s data (IsString s, Monoid s) => Attr s = Attr s s class (Monoid s, IsString s) => HTML d s | d -> s where toHtml :: d -> s instance (Monoid s, IsString s) => HTML (Dom s) s where toHtml (Element name attrs children) = mconcat [ fromString "<", name, mconcat $ map toHtml attrs, fromString ">", mconcat $ map toHtml children, fromString "</", name, fromString ">" ] toHtml (Text s) = s instance (Monoid s, IsString s) => HTML (Attr s) s where toHtml (Attr key value) = mconcat [ fromString " '", key, fromString "'='", value, fromString "' " ] -- Darrin

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